In powder molding, there may be a case where a mold referred to as a stepped die is used in molding an outer peripheral side of a part 31 having a step 30 on an outer periphery as shown in FIG. 8, for example. FIG. 9 is a plan view of one example of such a stepped die 21, and FIG. 10 is a cross-sectional view of the stepped die 21.
The stepped die 21 includes an inner ring 22 having a cylindrical shape, and an outer ring 23 having a cylindrical shape which is fitted on an outer periphery of the inner ring 22 by shrinkage fitting, and a recessed portion 24 for molding is formed on an inner side of the inner ring 22. The recessed portion 24 has a stepped portion 25 which corresponds to the step 30 of the part 31. As shown in FIG. 9, the stepped portion 25 has a rectangular shape as viewed in a plan view. A flange portion 27 which engages with a die plate 26 is formed on an outer periphery of the outer ring 23.
In molding the part 31 using the above-mentioned stepped die 21, after molding, the part 31 is removed from the stepped die 21 in such a manner that the stepped die 21 is lowered together with the die plate 26 so that the part 31 is pushed upward relative to the stepped die 21 by a lower punch 28 in a fixed state. Accordingly, a support which supports the stepped die 21 cannot be disposed in a space S below the stepped die 21 since the support becomes an obstacle against lowering of the stepped die 21. In view of the above, the compression of powder is performed using an upper surface 28a of the lower punch 28 and an upper surface 25a of the stepped portion 25 as pressure receiving surfaces in a state where only a flange portion 27 formed on an outer periphery of the stepped die 21 is supported and a lower surface of the stepped die 21 is not supported.
However, in such a pressure applying method, a pressure applied to the stepped portion 25 is received by an edge portion or a corner portion of the stepped portion 25 and hence, a bending stress is concentrated on the corner portion thus giving rise to a possibility that a crack C occurs (see FIG. 11). There is a possibility that the occurrence of the crack C not only leads to a rupture of the stepped die 21, but also influences accuracy of a finished part 31.
In view of the above, to prevent the occurrence of a crack by alleviating the stress concentration at the corner portion of the stepped portion of the stepped die, there has been proposed a method where a ring is mounted on an outer periphery of a die portion on which a bending stress acts by tight fitting (see patent literature 1).